In many optical local area networks (LANs) and optical backplanes that have a bus-type architecture, an optical repeater receives and combines optical signals from optical transmitters in system nodes, as well as amplifies and retransmits these signals back to optical receivers in the system nodes. The optical repeater utilizes optical couplers as a combiner and as a splitter. The combiner receives optical signals from the optical transmitters via individual optical fibers and combines those optical signals. The splitter transmits a portion of a common optical signal (generated by the optical repeater) to each optical receiver via an individual optical fiber. The optical splitter uses a single optical source to generate an optical signal for transmission on optical fibers connected to the optical receivers.
Optical couplers for performing the functions of optical combiners and splitters are known in the art. One such optical coupler is disclosed in U.S. Pat. No. 4,913,508 of L. L. Blyler, Jr. et al., issued Apr. 3, 1990. When communicating light between a first plurality and a second plurality of optical fibers, this prior art optical coupler depends on the optical core of the optical coupler bending the light sufficiently so that higher order modes of light are obtained. These higher order modes of light then produce a uniform amount of light striking each of the receiving optical fibers. Such prior art optical couplers having reasonable lengths perform adequately for optical sources which produce a rapidly diverging beam of light. For example, as illustrated in the aforementioned U.S. Pat. No. 4,913,508, one such source is a large numerical aperture step index optical fiber.
Although the prior art optical couplers perform adequately for certain optical sources, a problem arises when certain types of optical lasers are used as optical sources in such couplers because certain optical lasers have a narrow divergence angle. This problem can arise even when the laser is connected to the optical coupler via an optical fiber. For a diode laser transmitting into a transparent medium, this angle is typically a 5 degree half angle in one dimension and a 15 degree half angle in the other. The optical beam of such a laser diverges so slowly that the prior art couplers must be made very long to achieve the required uniformity of mode distribution within the optical coupler. Such long prior art couplers are difficult to use because of the physical space required and because of the amount of light energy dissipated within such an optical coupler.